1. Field of the Invention
The present invention relates to a fuel electrode of a solid oxide fuel cell and a process for producing the same.
2. Description of the Prior Art
Fuel cells have recently been noted as energy sources from the viewpoint of not only saving of resources but also influence on the environment. In a solid oxide fuel cell (SOFC), a fuel electrode is disposed on one surface of a solid electrolyte layer, while an air electrode is disposed on an opposite surface. The cell is constituted of a plurality of cells electrically connected to one another, with separators each laminated between the adjacent cells for distributing fuel and oxidizing gas to the cell, and has a higher operation temperature of 700 to 1000xc2x0 C. than other fuel cells to provide a higher power generating efficiency. Since all constituting materials are solid, they can easily be handled. These advantages advance practical use.
A circuit structure of a conventional typical solid oxide fuel cell is diagrammatically shown in FIG. 1 together with a sectional view of a cell. In FIG. 1, a fuel electrode 2 is formed on one surface of a central solid electrolyte layer 1 (upper surface of the solid electrolyte layer 1 in FIG. 1), an air electrode 3 is formed on an opposite surface, and the electrodes have interfaces 4 and 5 with the solid electrolyte layer 1, respectively. In the solid electrolyte layer 1, mainly 8YSZ (YSZ denotes a yttria-stabilized zirconia) or 3YSZ is used. The fuel electrode 2 and air electrode 3 are connected by an external circuit via a load 6. When fuel gases such as hydrogen (H2), methane (CH4) and the like are supplied to the fuel electrode 2, and oxidizing agents such as air, oxygen (O2) and the like are supplied to the air electrode 3, an electromotive force is generated between the electrodes, so that current flows to the load 6 connected to the external circuit. It is known that in this type of solid oxide fuel cell, electrode material composition and structure have a large influence on cell performance, and especially the influence of the fuel electrode 2 is large. In the fuel electrode 2, a cermet of metal and oxide is generally used.
As the fuel electrode 2, a mixture of Ni and YSZ particles, i.e., Nixe2x80x94YSZ cermet, is known.
Since the electrode reaction of the solid oxide fuel cell is accompanied with a gas phase reaction, as shown in FIG. 2, the conventional electrode reaction field of the fuel electrode 2 is limited to the vicinity of three-phase lines in which the electrolyte, Ni particles in the cermet and gas phase coexist, the current is drawn to the vicinity of the three-phase lines, and current density locally increases. Moreover, in the conventional fuel electrode 2, Ni particles are not uniformly dispersed in the cermet, and electrolyte films or other fine particles are not present in a small amount on surfaces of the Ni particles. Therefore, it is difficult to inhibit the Ni particles from being flocculated.
As a result, a solid oxide fuel cell having the conventional fuel electrode has the disadvantages that (1) since the electrode reaction of the fuel electrode is limited to the vicinity of the three-phase lines with the Ni particles in the cermet and the gas phase coexisting therein, the overvoltage is large, and (2) the flocculation of the Ni particles directly causes the deterioration of the fuel cell.
The present invention has been developed to solve the above-mentioned problems in the conventional fuel electrode, and an object thereof is to provide a fuel electrode of a solid oxide fuel cell and a producing process thereof, in which, by providing metal oxide particles in a cermet with electronic conductivity, the field of electrode reaction is largely enlarged, flocculation of Ni particles is minimized, and power generating performance of the cell can be enhanced.
To attain the above-mentioned object, according to a first basic aspect of the present invention, there is provided a process for producing a fuel electrode of a solid oxide fuel cell, each cell comprising a solid electrolyte layer, a fuel electrode disposed on one surface of the solid electrolyte layer, and an air electrode disposed on an opposite surface, by alternately laminating a plurality of cells, adjacent cells being electrically connected to each other, and a plurality of separators for distributing fuel gas to the fuel electrode of each cell and oxidizing gas to the air electrode, comprising the steps of: adding a solution of a metallo-organic compound of yttrium (Y) and a solution of a metallo-organic transition-metal compound to a solution of a metallo-organic compound of zirconium (Zr) to prepare a mixed solution of metallo-organic compounds of Zrxe2x80x94Y-transition metal; mixing NiO powder into the mixed solution of the metallo-organic compounds to prepare a slurry; and successively subjecting the slurry to hydrolysis, polycondensation, pyrolysis, annealing and reduction to obtain a cermet formed of yttria-stabilized zirconia (YSZ, i.e., CeO2xe2x80x94Y2O3xe2x80x94ZrO2) containing the transition metal dissolved therein, and having electronic conductivity in a fuel electrode operating atmosphere, and Ni.
According to a second basic aspect of the present invention, there is provided a process for producing a fuel electrode of a solid oxide fuel cell, each cell comprising a solid electrolyte layer, a fuel electrode disposed on one surface of the solid electrolyte layer, and an air electrode disposed on an opposite surface, by alternately laminating a plurality of cells, adjacent cells being electrically connected to each other, and a plurality of separators for distributing fuel gas to the fuel electrode of each cell and oxidizing gas to the air electrode, comprising the steps of: adding a solution of a metallo-organic compound of yttrium (Y) and a solution of a metallo-organic transition-metal compound to a solution of a metallo-organic compound of zirconium (Zr) to prepare a mixed solution of metallo-organic compounds of Zrxe2x80x94Y-transition metal; mixing NiO powder and cerium oxide powder containing a divalent or trivalent metal oxide dissolved therein to the mixed solution of the metallo-organic compounds to prepare a slurry; and successively subjecting the slurry to hydrolysis, polycondensation, pyrolysis, annealing and reduction to obtain a cermet formed of yttria-stabilized zirconia (YSZ) containing the transition metal dissolved therein, nickel (Ni) and cerium oxide containing the divalent or trivalent-metal dissolved therein.
The transition metal described in the first and second basic aspects is one selected from the group consisting of cerium (Ce), titanium (Ti) and praseodymium (Pr).
The metallo-organic compound described in the first and second basic aspects is one selected from the group consisting of metallic octylate, metallic naphthenate, metallic stearate and another metallic aliphatic acid salt, and metallic acetyl acetonate complex.
The fuel electrode described in the first and second basic aspects is formed on a solid electrolyte by a screen printing process.
The concentration of the transition metal in the yttria-stabilized zirconia (YSZ) containing the transition metal dissolved therein according to the first and second basic aspects is in the range of 1 mol % to 30 mol %.
The volume fraction of the cerium oxide containing the divalent or trivalent metal dissolved therein according to the second basic aspect is in the range of 1% to 70%.
The concentration of Ni in the fuel electrode according to the first and second basic aspects is in the range of 20% to 95% as a volume fraction.
The concentration of the yttria-stabilized zirconia containing the transition metal dissolved therein in the cermet according to the first and second basic aspects is in the range of 1% to 50% as the volume fraction.
The divalent or trivalent metal oxide according to the second basic aspect is one, or a combination of plural ones, of BeO, MgO, CaO, SrO, BaO, Sm2O3, Y2O3, La2O3, Gd2O3, Sc2O3, Pr2O3, Nd2O3, Eu2O3, Yb2O3, Dy2O3, and Ho2O3.
The cermet according to the second basic aspect has a structure in which surfaces of Ni particles and surfaces of cerium oxide particles containing divalent or trivalent metals dissolved therein are covered with YSZ containing the transition metal dissolved therein in the form of thin films or fine particles.
The hydrolysis of the first or second basic aspect is performed using moisture in air.
As materials of the cermet according to the second basic aspect, cerium oxide powder containing the divalent or trivalent metal dissolved therein, Ni powder and metallic octylate solution of Ce, Y and Zr are used, and YSZ fine particles containing a fine transition metal dissolved therein are uniformly dispersed between the cerium oxide particles containing the divalent or trivalent metal dissolved therein and the Ni particles. In this case, the average particle diameter of the Ni particles in the cermet is 1 xcexcm or more, the average particle diameter of the cerium oxide particles containing the divalent or trivalent metal dissolved therein is 1 xcexcm or more, and the average particle diameter of the YSZ particles containing the transition metal dissolved therein is 1 xcexcm or less.
According to a third basic aspect of the present invention, there is provided a fuel electrode of a solid oxide fuel cell comprising a cermet composed of yttria-stabilized zirconia (YSZ), containing a transition metal dissolved therein and having electronic conductivity in a fuel electrode operating atmosphere, and nickel (Ni).
The cermet mentioned in the third basic aspect has a structure in which Ni particles and YSZ particles containing the transition metal dissolved therein are uniformly dispersed.
According to a fourth basic aspect of the present invention, there is provided a fuel electrode of a solid oxide fuel cell comprising a cermet composed of yttria-stabilized zirconia (YSZ) containing a transition metal dissolved therein and having electronic conductivity in a fuel electrode operating atmosphere, nickel (Ni), and cerium oxide containing a divalent or trivalent metal dissolved therein.
According to the fourth basic aspect, the cermet has a structure in which surfaces of Ni particles and surfaces of cerium oxide particles containing the divalent or trivalent metal dissolved therein, uniformly dispersed in the cermet, are covered with YSZ thin films or fine particles containing the transition metal dissolved therein.
According to the fourth basic aspect, as materials of the cermet, cerium oxide powder containing a divalent or trivalent metal dissolved therein, Ni powder and a metallic octylate solution of Ce, Y and Zr are used, and YSZ fine particles containing the transition metal dissolved therein are uniformly dispersed between the cerium oxide particles containing the divalent or trivalent metal dissolved therein and the Ni particles. In the cermet, the average particle diameter of the Ni particles is 1 xcexcm or more, the average particle diameter of the cerium oxide particles containing the divalent or trivalent metal dissolved therein is 1 xcexcm or more, and the average particle diameter of the YSZ particles containing the transition metal dissolved therein is 1 xcexcm or less.
As clearly seen from the above-mentioned aspects, the present invention is mainly characterized in that:
(a) the transition metal for use in the fuel electrode is cerium (Ce), titanium (Ti) or praseodymium (Pr) which easily provides YSZ of the fuel electrode with the electronic conductivity;
(b) the metallo-organic compound for use in the fuel electrode is a metallic octylate, metallic naphthenate, metallic stearate or another metallic aliphatic acid salt, or metallic acetyl acetonate complex which is relatively stable as compared with other metallo-organic compounds; and
(c) the fuel electrode is formed on the solid electrolyte layer by a screen printing process.
In the present invention, pyrolysis of the metallo-organic compound as an oxide film forming process is applied. For example, since the pyrolysis as the film forming process is applied in synthesizing CeYSZ, a fuel electrode can be obtained with a structure in which CeYSZ films or fine particles are uniformly deposited on surfaces of Ni particles and particles of cerium oxide containing the divalent or trivalent metal dissolved therein or another metal oxide. Therefore, metal and metal oxide particles are uniformly dispersed without being flocculated.
The present invention having the above-mentioned aspects provides the following effects:
(1) Since cerium oxide (CeO2) is applied to the fuel electrode formed of the cermet of Ni and YSZ, as materials of a YSZxe2x80x94CeO2 solid-solution (CeYSZ), metallo-organic compounds are used to perform hydrolysis and polycondensation reactions in the slurry with Ni, so that CeO2 is dissolved in YSZ, and electronic conductivity is developed in CeYSZ. The electrode reaction is not limited to the vicinity of the three-phase lines of the solid electrolyte layer, Ni particles and gas phase, but also occurs in a three-phase interface of the solid electrolyte layer, YSZ with CeO2 doped therein (CeYSZ) and gas phase. Therefore, the field of electrode reaction is remarkably expanded. Moreover, when the electrode is produced in the above-mentioned process, fine particles of YSZ with CeO2 doped therein are uniformly dispersed around the Ni particles, so that the field of electrode reaction is further expanded.
(2) Reduction of Contact Resistance between the Electrode and Solid Electrolyte Layer
According to the present invention, a fuel electrode can be obtained with a structure in which the films or the fine particles of the electrolyte provided with electronic conductivity are uniformly deposited on the surfaces of Ni or NiO particles or a surface of the solid electrolyte layer as a center, so that the Ni or NiO particles are uniformly dispersed without being flocculated. Moreover, since the pyrolysis of the film forming process is applied in synthesizing CeYSZ, the solid electrolyte layer as the center near the interface and the electrolyte provided with electronic conductivity in the fuel electrode are bound with each other very strongly, and integrally constructed. Therefore, the electrolyte provided with electronic conductivity of the fuel electrode grows from the surface of the central solid electrolyte layer, and the field of the electrode reaction is enlarged. Consequently, there is provided an electrode small in contact resistance.
(3) Reduction of Polarization
As described in (2) above, since the Ni or NiO particles are uniformly dispersed in the cermet, and the electrolyte particles in the cermet provided with the electronic conductivity bind strongly with the solid electrolyte layer in the interface, the electrode reaction is increased in the structure. Specifically, since an ideal electrode structure in which the electrode reaction easily occurs is obtained, polarization by the fuel electrode is remarkably minimized.
(4) Enhancement of Binding Strength of Central Solid Electrolyte Layer and Electrode
As described in (2) above, since the central solid electrolyte layer strongly binds with the electrolyte layer of the fuel electrode, the binding strength of the electrode and solid electrolyte layer is very strong, and the Ni particles are uniformly dispersed. The periphery of the Ni particles is covered with the electrolyte film or other fine particles, the Ni particles are prevented from being flocculated, and the electrode is prevented from being easily peeled. As a result, a long-life electrode is obtained.
Moreover, according to the present invention, the solution of the metallo-organic compound of yttrium (Y) and the solution of the metallo-organic transition-metal (M) compound are added to the solution of the metallo-organic compound of zirconium (Zr) to prepare the mixed solution of Zr, Y and M. Furthermore, Nio powder is mixed with the cerium oxide powder (e.g., SDC powder) containing the divalent or trivalent metal oxide, e.g., one or a combination of two or more of oxides of yttrium (Y) and lanthanoids (La, Nd, Sm, Gd, Dy, Ho, Yb and the like) dissolved therein to prepare the slurry. In the slurry, hydrolysis, polycondensation, pyrolysis, annealing and reduction of the Zr, Y, M mixed salt are successively performed to obtain a cermet formed of the yttria-stabilized zirconia (YSZ) particles containing the transition metal (M) dissolved therein, nickel (Ni) particles, and cerium oxide particles containing one, or a combination of two or more, of oxides of yttrium (Y) and lanthanoids (La, Nd, Sm, Gd, Dy, Ho, Yb and the like) dissolved therein. Therefore, remarkably superior effects are provided as follows:
The field of the electrode reaction can further be enlarged;
Since the electrode having excellent electrode performance can be obtained at either high or low operation temperature, the solid oxide fuel cell whose performance is not impaired by the temperature distribution in the cell can be produced; and
Since the CeO2 particles containing the divalent or trivalent metal oxide dissolved therein and CeYSZ fine particles inhibit the Ni particles from being flocculated with one another, a fuel electrode difficult to be deteriorated and high in durability can be obtained.
These and other objects, features and advantages of the present invention will be appreciated by a person skilled in the art in the following description of preferred embodiments conforming to the principle of the present invention in conjunction with the attached drawings.